Use dnsresolver_aidl_interface-unstable-cpp in test

The netd_benchmark test should use latest IDnsResolver interface, rather
than last fixed version. Otherwise, the new added stuff in IDnsResolver
will cause build error.

Bug: 147488977
Test: atest under /system/netd
Change-Id: Ie7f0cb9000708cdbea2b5bd6aa6d39983abc302b